I seem to have cut up the wrong debit card.
You see I have 2 debit cards: one for our joint account and one that is mine, all mine. When a new card arrived and Husband gave it to me to sign I assumed it was to replace our joint card. But as I discovered when I tried to get some money out and it refused to recognise my PIN, it wasn't.
I'm going to have to explain this to the bank now.